A method and apparatus for the making of covers with a foil is known from patent application WO 2006/042426 A1. The known apparatus has a conveyor device and several processing stations and a testing station. The conveyor device delivers cover blanks to the individual processing stations and to the test station. In a first processing station, a cover blank is created with a completely surrounded opening, insofar as by a stamping process an opening is stamped into a disc. In the next processing station, the inner edge of the cover blank is drawn downwardly. In the following processing station a foil having a flap is placed over the surrounded opening of the cover blank and is fastened to the blank by heat sealing. The foil for this is provided on its underside with a layer of plastic material and is stamped from a broad foil web. The foil is then placed over the surrounded opening, which can also be referred to as a middle recess, and is pressed to the edge of the surrounded opening under the effect of heating so that the foil is sealingly connected to the cover blank by the melting and subsequent cooling of the plastic material layer. An additional processing station for cooling can be provided. Then in a further processing station, the foil is provided with an embossing and the inner edge of the cover blank now located below the foil is beaded. Finally, the now finished cover is subjected to a test in a testing station which testing includes a test of the sealing tightness of the foil applied to the cover.
As described in patent application WO 2006/053457 A2 the foil can be so sealed or adhered to the cover ring insofar as in a first step the sealing or adhering is partially accomplished and in a second step is completed, with by the second step the connection between the foil and the cover blank being entirely reinforced. The sealing or adhering takes place by heat effect. The partial sealing or adhering is so created that the sealing or adhesion takes place in only predetermined partial areas and/or because of insufficient heat effect is not entirely carried out. The temperature for the sealing or adhesion in the first step and/or in the second step has preferably a value of about 200° C.
Based on the principal of induction heating, it is known to create heat in an electrically conducting workpiece by means of an inductor which typically includes a U-shaped magnet core with two legs, with each leg having wound onto it an electrically conducting coil connected to a source of electric current. The legs of the U-shaped magnet core carrying the coils represent the magnet poles. The inductor thereby forms an open magnetic circuit. To heat a workpiece the poles are held close to the outer surface of the workpiece to be heated. With the supply of an alternating current, a magnetically alternating field is created under the influence of which in the outer surface of the workpiece high frequency eddy currents are created so that heat in the form of eddy current loses (so called “skin effect”) is created. This is for example used for the warming of a container wall (see U.S. Pat. No. 5,690,851 A). Corresponding inductors are for example also known from U.S. Pat. Nos. 5,101,086 A and 7,022,951 B1. The thinner the wall of a workpiece the greater must be the frequency of the alternating current source and with it the frequency of the created alternating magnetic field, so that the eddy currents created in the workpieces are close to a sufficiently large electrical resistance, so as to create sufficient heat by the eddy current losses.
From U.S. Pat. No. 4,740,663 A an inductive heating unit is known by means of which electrical energy can be induced into the metallic periphery of a metal foil carrier of a cover for a container so that the cover is heated and is heat connected with the container. The induction heating unit has a magnetic core with two opposed U-shaped sections with one coil being arranged inside of one section. The cover is clamped to the opposed legs of one side of the magnetic core, while an air gap is formed with the other side of the magnetic core. This provides a closed magnetic circuit with each of the opposed legs forming a pole. In the cover lying between the legs of one side of the magnetic core by eddy currents and magnetic reversing loss a partial heating is obtained. The cover which connects the two legs of one pole of one side of the magnetic core with one another so to speak short-circuits the magnetic field between the two poles of the magnet circuit. The known devices for inductive heating, which are also known as inductors, are supplied with high frequency energy and typically for each workpiece a certain size of inductor with suitable tolerances is required. Also different materials often require different types of inductors, as for example, is the case with workpieces made of aluminum in comparison to workpieces made of steel alloy, since this material especially strongly influences the coupling. If the inductor is to be used with an apparatus with a conveyor, it can happen that with known induction the typically metallic guide rails of the conveyor can come under the effect of an inductor arranged near a workpiece and can likewise become heated.